The phrase John the Baptist used is, “unto repentance” and you only see this phrase show up one other place in the word of God and it is dealing directly with a Jew living in a future tribulation which is followed by a Kingdom reign that has the Lord Jesus Christ reigning a thousand years on Earth, with Israel as the apple of his eye.
If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame. Hebrews 6:6
It is vital to note that Paul never used the phrase “unto repentance”. It’s always “unto salvation” with him! These defining and separating phrase, “unto repentance and “unto salvation” are important to noticing the distinction. Here are some examples of “unto salvation”.
For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. Romans 1:16
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. Romans 10:9-10
And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus. 2 Timothy 3:16
Who are kept by the power of God through faith unto salvation ready to be revealed in the last time. 1 Peter 1:5
It is simply not the same message or associated with the same gospel. Paul’s gospel and the gospel preached in the Gospels are just as separate as the law and grace, though both are concerning Christ. The gospel preached in the gospels was a gospel of repentance. It was unto repentance. The one that Believers preach is the gospel of grace, the one that Paul taught the collective Christian body of his day.